Not sure if you mean the actual little ad pop-up boxes, or just adverts in general... I watch* on average 25 Youtube videos per day and I'd estimate around 10 of those have pre-roll adverts, I normally watch 1 or 2 pre-roll adverts (assuming I have not seen them before) per day and click through after. The sidebar adverts I've never clicked and same goes for the little ones that slide in about 10 seconds into a video.
* some of the videos I "watch" are in the background (music) so I included those in my count but they rarely (if ever) have adverts (because I listen to independent music on Youtube)
Interesting thing I've noticed: At home, no video's got a pre-roll (Slovenia), but when I was in the US almost every video had a pre-roll ... it was so annoying I almost stopped using YouTube while I was in the US even though I watch probably 20+ videos per day at home.
Thing is, I watch mostly music videos and it's really really really really annoying to listen to a pre-roll first.
